1. Field of the Invention
The invention relates to coiled tubing mounted inspection tool systems, and more particularly, to a modular downhole inspection tool for selective attachment to the end of a length of coiled tubing. The invention may be practiced in connection with the maintenance and servicing of oil, gas, geothermal and injection wells.
2. History of the Prior Art
In the drilling and production of oil and gas wells, it is often necessary to obtain at the surface information concerning conditions within the borehole. For example, tools and other objects may become lodged in the borehole during the drilling of a well. Such objects must be retrieved before drilling can continue. When the removal of foreign objects from a borehole is undertaken, known as "fishing", it is highly desirable to know the size, position and shape of the obstructing object in order to select the proper fishing tool to grasp the object and remove it from the borehole. In addition, it is often desirable to confirm the operational condition of a piece of downhole production equipment, for example, whether or not the ports of a sliding sleeve valve are open or closed, in order to rely upon that condition in an operational procedure. Such information is very difficult to obtain because of the hostile downhole environment within a borehole filled with opaque drilling fluids.
In the operation and/or periodic maintenance of producing injection wells, it is also frequently necessary to obtain information about the construction and/or operating condition of production equipment located downhole. For example, detection of the onset of corrosion damage to well tubing or casing within the borehole enables the application of anti-corrosive treatments to the well. Early treatment of corrosive well conditions prevents the highly expensive and dangerous replacement of corrosion damaged well production components. Other maintenance operations in a production well environment, such as replacement of various flow control valves or the inspection of the location and condition of casing perforations, make it highly desirable for an operator located at the surface to obtain accurate, real-time information about downhole conditions. The presence of production fluids in the well renders accurate inspection very difficult.
Wireline tools, such as that shown in U.S. Pat. No. 3,401,749, have long been used in downhole environments including those which are mounted to a length of coiled tubing, as in U.S. Pat. No. 4,877,089, and to such tubing through a coaxial coiled tubing cable head, as in U.S. Pat. No. 4,941,349. Moreover, the use of coiled tubing to position a downhole tool and enable the accurate orientation of such a tool is shown in U.S. Pat. No. 4,685,516. Various related techniques have been proposed for obtaining at the surface information about the conditions within a borehole. One approach has been to lower an inspection device, such as an optical or acoustical sensor positioned on the end of a section of coiled tubing, into the borehole and produce a slug or "bubble" of optically transparent and/or acoustically homogenous fluid within the borehole to enable the accurate inspection by the inspection sensor attached to the lower end of the tubing. Such a system is shown in U.S. Pat. No. 4,938,060 to Sizer et al. and assigned to the assignee of the present invention.
In addition, in the case of optical inspection sensors of the type shown in the Sizer et al. patent, it is also desirable to provide a means for simultaneously cooling the downhole sensor equipment as well as injecting the optically transparent and/or acoustically homogenous fluid within the borehole which enhances the observation and inspection functions performed by the equipment. Such a system is shown in parent U.S. patent application Ser. No. 07/703,287, filed May 20, 1991 and assigned to the assignee of the present invention.
In systems such as that shown in the parent application hereto, a television camera is mounted to the lower end of a length of coiled tubing and connected to monitoring and recording equipment at the surface by a transmission line such as a fiberoptic or coaxial cable. Coiled tubing units represent a very large capital investment and the operator of such a unit may desire to use the unit for other applications, such as the injection of nitrogen or other fluids into a wellbore or the attachment of standard electric logging tools, as well as a downhole inspection system. In such cases, providing the downhole inspection equipment as a modular unit which may be selectively connected and disconnected from the coiled tubing would be highly desirable. Further, providing modular downhole inspection equipment which is fluid and pressure sealed from the interior of the length of coiled tubing would provide advantages in the event the inspection module was stuck or lodged downhole and it became necessary to separate the coiled tubing from it in order to first remove the coiled tubing and then recover the lodged equipment with a fishing tool. In addition, for a separable inspection module in which a data transmission means incorporating a coaxial fiberoptic cable is employed to couple the signal from the inspection tool to the surface, sealing of the lower end of the optical fibers of the cable from pressurized liquids within the borehole when the tool is separated is essential in order to prevent a substantial loss of expensive cable due to intrusions of fluid into the cable fibers due to capillary action.
It would be an improvement in coiled tubing mounted downhole inspection systems if a fluid pressure sealed and transmission line equipped coiled tubing module unit could be plug connectable to the inspection imaging and electronics modules to allow multiple uses of the coiled tubing module as well as to minimize damage to the equipment in the event the tool becomes lodged downhole and necessitates separation of the tubing module from the other downhole equipment. In addition, it would be desirable to provide such a modular inspection system which allows threading of a sealed coiled tubing cable head module through a coiled tubing injector prior to plug connection of the electronics and imaging modules of the inspection tool.